Rinsable cold head for a cryo refrigerator using the pulse tube principle

ABSTRACT

A cold head ( 20; 30 ) for a cryo refrigerator comprises at least one refrigerator tube ( 5 ), at least one pulse tube ( 6, 9 ), and one feed line ( 4 ) for feeding a working gas from a turning valve ( 3 ) of the cryo refrigerator to a warm end of the regenerator tube ( 5 ), wherein a cold end of the pulse tube ( 6, 9 ) is connected to a cold end of the regenerator tube ( 5 ) via a connecting line ( 7, 8 ), and wherein a buffer line ( 10, 11 ) extending to a buffer volume ( 14, 15 ) of the cryo refrigerator is provided at a warm end of the pulse tube ( 6, 9 ), is characterized in that a first reversing valve ( 21 ) is provided in the feed line ( 4 ), and a second reversing valve ( 23; 31, 32 ) is provided in the buffer line ( 10, 11 ), such that, in a rinsing state of the reversing valves ( 21, 23; 31, 32 ), the turning valve ( 3 ) and the buffer volume ( 14, 15 ) are separated from the regenerator tube ( 5 ) and from the pulse tube ( 6, 9 ), and the regenerator tube ( 5 ) and the pulse tube ( 6, 9 ) can be rinsed via the reversing valves ( 21, 23; 31, 32 ). The inventive cold head can be cleaned in the cold, installed state in the cryostat.

This application claims Paris Convention priority of DE 10 2006 054 668.7 filed Nov. 17, 2006 the complete disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a cold head for a cryo refrigerator, the cold head comprising:

-   -   at least one refrigerator tube,     -   at least one pulse tube,     -   a feed line for supplying a working gas from a turning valve of         the cryo refrigerator to a warm end of the regenerator tube,         wherein a cold end of the pulse tube is connected to a cold end         of the regenerator tube via a connecting line, and wherein a         buffer line is provided at a warm end of the pulse tube, which         extends to a buffer volume of the cryo refrigerator.

A cold head of this type is e.g. disclosed by the commercially distributed model “PT405” by Cryomech, Inc., Syracuse, N.Y., USA, with operating instructions dated 2005.

Superconducting magnet configurations which are used e.g. in nuclear magnetic resonance (NMR) spectroscopy and imaging magnetic resonance (MRI) must be cooled to cryogenic temperatures (<100K). The superconducting magnet coils are usually disposed within a cryostat in a tank with liquid helium (of a temperature of around 2 to 4 K). The helium must be cooled due to unavoidable heat input.

In particular, cold heads which are operated in accordance with the pulse tube principle are used for this purpose. Cold is thereby generated through expansion of helium gas and performance of mechanical work through helium gas. Within the scope of the second effect, a shock wave is generated in the helium gas which is pushed through the helium gas, typically by a pulse tube in a buffer volume.

In prior art, a feed line of the cold head is alternately connected to a high-pressure connection (approximately 30 bar) via a turning valve, and to a low-pressure connection (approximately 5 bar) of a helium compressor. During connection to the high-pressure connection, helium gas flows through the feed line into the cold head, and during connection to the low-pressure connection, helium gas flows out of the cold head through the feed line. It is thereby possible to obtain temperatures below the boiling temperature of helium at normal pressure (approximately 4.2 K).

The cold head is soiled with time due to foreign gases such as oxygen, nitrogen, water vapor or hydrocarbons. These foreign gases may be contained in the helium gas in the form of impurities, penetrate into the gas lines through diffusion or enter the cooling cycle through the compressor oil. The foreign gases freeze out in the cold head, thereby impairing both the flow of the helium working gas and the heat exchange. The cooling capacity of the cold head decreases with increasing soiling of the cold head, until cleaning is finally necessary.

The cold head is conventionally cleaned through heating the cold head to room temperature and pumping out the impurities. The cold head is subsequently filled with highly pure helium.

For heating the cold head, the cold head must be removed from the cryostat of the superconducting magnet configuration. Towards this end, most magnet configurations must be “disconnected from the field”.

Discharging and charging of the superconducting magnet configuration is, however, time consuming (sometimes several days) and expensive (due to helium consumption). Moreover, there is the danger of a quench of the magnet configuration, which consumes very large amounts of helium and can damage the magnet configuration.

It is the underlying purpose of the present invention to present a cold head which can be cleaned in the installed, cold state in the cryostat.

SUMMARY OF THE INVENTION

This object is achieved by a cold head of the above-mentioned type, which is characterized in that a first reversing valve is provided in the feed line, and a second reversing valve is provided in the buffer line, such that in a rinsing state of the reversing valves, the turning valve and the buffer volume are separated from the regenerator tube and from the pulse tube, and the regenerator tube and the pulse tube can be rinsed via the reversing valves.

By means of the reversing valves, the inventive cold head can be connected to a rinsing gas, which is typically identical to the working gas (in each case preferably highly pure helium with a purity of approximately 5.9 or higher), and a rinsing gas flow through the cold head, in particular through its coldest areas, can be adjusted. Rinsing gas is introduced at a reversing valve and at the same time, rinsing gas flows out at the other reversing valve. The rinsing gas carries along foreign gases from the inside of the cold head and guides them out of the cold head. Thermal energy can also be supplied to the inner walls of the cold head together with the passing rinsing gas, such that frozen foreign gases are thawed and released.

The constructional principle of a conventional cold head, however, resembles a dead end. Passage is not possible. The only alternative is pumping out, whereby frozen foreign gases cannot be removed. Heating of the cold head is hardly possible.

One embodiment of an inventive cold head is particularly preferred, which is characterized in that the cold head has a heating means, in particular, an electric heating means. The heating means can heat the inner walls of the cold head and/or the rinsing gas in order to facilitate thawing of foreign gases in the cold head during cleaning. The heating means preferably acts at those locations in the cold head which are most likely to be clogged by foreign gases, such as narrow passages and bends in the gas lines and very cold areas.

In one preferred embodiment, the cold head comprises two cooling stages, which renders the cooling head more powerful. In this case, typically also two pulse tubes, two buffer lines with one reversing valve each, and two buffer volumes are provided.

In one particularly preferred embodiment, at least one reversing valve opens into a pressure-relief valve, wherein this pressure-relief valve is preferably adjusted to a mean working pressure of the cryo refrigerator. The pressure-relief valve helps to prevent impurities inside the cold head and in the rinsing lines. The mean working pressure of the cryo refrigerator results from the mean value of the helium pressure values at the high-pressure connection and low-pressure connection.

The invention also includes a method for cleaning a cold head of a cryo refrigerator, which works in accordance with the pulse tube principle, in particular, wherein the cold head is designed as described above in accordance with the invention, characterized in that, in the cold state of the cold head, the cold head is separated from the other components of the cryo refrigerator and rinsing gas flows through it. The passing flow may blow foreign gases out of the cold head. The other components typically comprise the turning valve including compressor, and optionally also the buffer volumes.

The rinsing gas preferably introduces thermal energy into the gas lines of the cold head. This enforces thawing and release of the foreign gases. The incoming rinsing gas is preferably warmer than the areas of the cold head to be cleaned, and the flow of the rinsing gas is adjusted such that the outflowing rinsing gas is still warmer than the areas of the cold head to be cleaned, which ensures that the rinsing gas can also introduce thermal energy into the areas to be cleaned. Rinsing gas at room temperature is generally sufficiently warm for cleaning a cold head.

Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used in accordance with the invention individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention.

The invention is shown in the drawing and is explained in more detail below with reference to embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a cryo refrigerator with cold head according to prior art, with two cooling stages;

FIG. 2 shows a schematic view of a cold head for a cryo refrigerator in accordance with the present invention, with one cooling stage;

FIG. 3 shows a schematic view of a cold head for a cryo refrigerator in accordance with the present invention, with two cooling stages.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically shows the circuit of a cryo refrigerator with a two-stage cold head 1 in accordance with prior art.

A compressor 2 pumps a working gas, which is at room-temperature, in the present case highly pure helium, from a low pressure side ND to a high pressure side HD. A motor-controlled turning valve 3 alternately connects (with a frequency of approximately 0.5 to 5 Hz) the high-pressure side HD (with approximately 30 bar) and the low-pressure side ND (with approximately 5 bar) to a feed line 4 of the cold head 1.

The feed line 4 represents the only helium connection of the cold head 1. It extends to an area RT of the cold head 1, which is at room temperature.

The feed line 4 feeds helium into the warm end of a refrigerator tube 5. From there, helium is guided at a first cold stage K1, which is at a temperature of between 40 and 80 K, to the cold end of a first pulse tube 6 with a first connecting line 7. Helium is guided from a second cold stage K2, which is at a temperature of approximately 2 to 5 K, to the cold end of a second pulse tube 9 via a second connecting line 8. The warm ends of the pulse tubes 6, 9 are finally connected to buffer volumes 14, 15 via buffer lines 10, 11 with throttle valves 12, 13 in each case. The buffer volumes 12, 13 may also be disposed outside of the cold head 1. The buffer lines 10, 11 are moreover connected to throttle valves 16, 17 and to the feed line 4. The throttle valves 12, 13, 16, 17 adjust the helium flows in the cold head for optimum cooling performance. The throttle valves of commercial devices are generally designed as non-adjustable, preset narrow passages.

The conventional cold head 1 is designed as a dead end for the working gas, and, in particular, cannot be rinsed.

FIG. 2 schematically shows an inventive, one-stage cold head 20 for a cryo refrigerator. Mainly the differences with respect to the conventional cold head of FIG. 1 are explained below.

The cold head 20 has a feed line 4 which is connected to a compressor for the working gas (in the present case highly pure helium, purity of 5.9 or more) via a turning valve (shown in FIG. 1; not shown in FIG. 2 for reasons of simplicity). The feed line 4 extends, in turn, to an area RT of the cold head 20, which is at room temperature.

A first reversing valve 21 is disposed in the feed line 4. In a first (not shown) position (cooling operating state) of the reversing valve 21, the warm end of a refrigerator tube 5 is connected to the feed line 4, and a rinsing line 22 is separated. In a second position (shown in FIG. 2) (rinsing state) of the first reversing valve 21, the warm end of the refrigerator tube 5 is connected to the rinsing line 22, and the feed line 4 (and thus also the turning valve associated with the compressor and the compressor) are separated from the refrigerator tube 5.

The refrigerator tube 5 is connected at the cooling stage K (at approximately 40 K) to the cold end of a pulse tube 6 via a connecting line 7. The warm end of the pulse tube 6 is connected to a buffer volume (or buffer container) 14 via a buffer line 10 and a throttle valve 12. The buffer volume 14 may also be arranged externally (i.e. outside of the cold head 20).

A second reversing valve 23 is disposed in the buffer line 10. The warm end of the pulse tube 6 is connected to the buffer volume 14 in a first (not shown) position (cooling operating state) of the second reversing valve 23, and a rinsing line 24 is separated. In a second position (shown in FIG. 2) (rinsing state), the warm end of the pulse tube 6 is connected to the second rinsing line 24. In contrast thereto, the buffer volume 14 is separated from the pulse tube 6. The buffer line 10 and feed line 4 are connected via a throttle valve 16.

In the rinsing state, a rinsing gas (in the present case highly pure helium, purity 5.9 or more), which is stored e.g. in a compressed gas bottle (not shown), can be guided through the first rinsing line 22 through the cold head 20 (see arrows). For rinsing, a high rinsing gas throughput is preferably selected (a considerably higher gas turnover compared to the time average in the feed line 4 in one direction in cooling operation). With high throughput, the rinsing gas is not cooled down to the temperature of the cooling stage K during passage through the cooling stage K, such that the cold head 20 in the area of the cooling stage K can be sufficiently heated on the inner sides of the gas lines, although the cold head 20 is still installed in the cryostat and is therefore cooled from the outside by the liquid helium stored in the tank. Foreign gas is carried along in the working gas lines of the cold head 20 during passage of rinsing gas, thereby cleaning the cold head 20.

A pressure-relief valve 25 is disposed on the second rinsing line 24, which is preferably adjusted to the medium working pressure of the cryo refrigerator, in the present case (30 bar+5 bar)/2=17.5 bar. Rinsing gas that has passed through the cold head 20 flows out via the pressure-relief valve 25. It is clear that the inlet pressure of the rinsing gas at the first reversing valve 21 must be sufficiently larger than the adjusted opening pressure of the pressure-relief valve 25. The rinsing gas that flows out at the pressure-relief valve, can be collected if desired.

In their respectively adjusted connecting direction, the reversing valves 21, 23 have a negligible flow resistance for rinsing gas (or working gas) compared to the remaining cold head 20.

Within the scope of the invention, the rinsing gas may flow from the refrigerator tube 5 to the pulse tube 6 or vice versa. In the latter case, the pressure-relief valve 25 must be connected to the first rinsing line 22 in contrast to FIG. 2, and the bottles containing compressed helium must be connected to the second rinsing line 24.

FIG. 3 schematically shows an inventive two-stage cold head 30. The structure is similar to the structure of FIGS. 1 and 2. Mainly the differences are explained.

The feed line 4 extends via a first reversing valve 21 into the warm end of the refrigerator tube 5 in the area RT of the cold head 30, which is at room temperature, as in the embodiment of FIG. 2. The refrigerator tube 5 is connected to the pulse tubes 6, 9 at the cooling stages K1 at a temperature of 40 to 80 K and K2 at a temperature of 2 to 5 K via connecting lines 7, 8. The pulse tubes 6, 9 are connected to the buffer volumes 14, 15 via buffer lines 10, 11. Each buffer line 10, 11 has one second reversing valve 31, 32 with one rinsing line 33, 34 and one pressure-relief valve 25 each.

Rinsing gas can again be introduced via the first rinsing line 22 in the illustrated flow-through state of the reversing valves 21, 31, 32, which flows through the refrigerator tube 5 and the two pulse tubes 6, 9 and is discharged via the rinsing lines 33, 34 or their pressure-relief valves 25.

Stop valves (not shown) may additionally be provided in each rinsing line 33, 34 in order to limit a rinsing process or passage to one of the pulse tubes 6, 9 (or one side of the cold head 30). Towards this end, one stop valve is opened and the other is closed. The pulse tubes 6, 9 are sequentially rinsed. This prevents irregular cleaning with simultaneous flow through both pulse tubes 6, 9, and improves control of the cleaning process.

Within the scope of the invention, the temperature of at least one cooling stage (e.g. K, K1, K2), in particular of the second cooling stage, may also be monitored, and cleaning of the cold head can be automatically initiated when a limit temperature has been exceeded during cooling operation. The reversing valves can then be actuated in an electronically controlled fashion.

In summary, the invention proposes a cold head for a cryo refrigerator that works in accordance with the pulse tube principle, with at least two connections for rinsing the cold head with rinsing gas at least in those areas where impurities tend to be deposited. Rinsing improves the cleaning effect compared to pumping out, via only one inlet, in particular, since the passing rinsing gas also easily heats the areas or line walls to be cleaned. In this case, the cold head can also be effectively cleaned in its cold state, and the cold head need not be removed from a cryostat for cleaning. 

I claim:
 1. A cold head for a cryo refrigerator, the cold head comprising: at least one refrigerator tube, said refrigerator tube having a warm end and a cold end; at least one pulse tube, said pulse tube having a warm end and a cold end; a cryo refrigerator turning valve; a feed line for feeding a working gas from said turning valve to said warm end of said refrigerator tube a connecting line connecting said cold end of said pulse tube to said cold end of said refrigerator tube; means defining a cryo refrigerator buffer volume; a buffer line extending from said warm end of said pulse tube to said buffer volume; a first reversing valve disposed in said feed line; and a second reversing valve disposed in said buffer line, wherein, in a rinsing state of said first and said second reversing valves, said turning valve and said buffer volume are separated from said refrigerator tube and from said pulse tube, wherein said regenerator tube and said pulse tube can be rinsed via said first and said second reversing valves.
 2. The cold head of claim 1, wherein the cold head comprises a heating means or an electric heating means.
 3. The cold head of claim 1, wherein the cold head has two cooling stages.
 4. The cold head of claim 1, wherein at least one of said first and said second reversing valves opens into a pressure-relief valve.
 5. The cold head of claim 4, wherein said pressure-relief valve is adjusted to an average working pressure of the cryo refrigerator.
 6. A method for cleaning the cold head of claim 1, wherein, in a cold state of the cold head, the cold head is separated from other components of the cryo refrigerator and a rinsing gas flows through the cold head. 